This invention relates to production of aluminum, and more particularly it relates to a treatment for carbonaceous members such as carbon blocks and carbon cathodes for use in the production of aluminum to improve performance of the cell.
In U.S. Pat. No. 5,961,811, there is described the Hall-Heroult process for making primary aluminum from aluminum oxide dissolved in a molten salt such as cryolite. In that patent, electrolysis is used to form molten aluminum at the cathode. The electrolysis is carried out at a temperature in the range of about 930 to 980xc2x0 C. The molten salt is contained in a steel shell which is lined with refractories and carbonaceous material. The lining containing the cathode metal, located in the bottom of the cell, is usually made of carbon materials. In addition, refractories are used to maintain thermal conditions in the cell. The amount of carbon used is substantial. For example, a Hall-Heroult cell of moderate size uses about 24,000 pounds of carbon block for lining purposes and uses about 10,000 pounds of carbon ramming paste to complete the lining and to hold the carbon blocks in place. The cell has to be relined about every 4 to 6 years, producing large quantities of used carbonaceous material and refractories, i.e., spent potlining.
As noted in U.S. Pat. No. 5,961,811, the use of carbonaceous cathodes is not without problems. For example, they are not readily wettable with molten aluminum. Thus, conductivity through the surface of the cathode is not uniform but tends to be intermittent. Also, the carbon cathode surface reacts with the molten aluminum to form aluminum carbide which depletes the cathode at a rate of 2 to 5 cms per year for an operating electrolytic cell. This depletion is fostered by the presence of sludge containing fluoride bath components at the interface between cathode carbon and metal. The aluminum carbide also is detrimental because it results in a high electrical resistivity material which interferes with the efficiency of the cell.
The carbon cathodes have another problem. The presence of sodium results in the formation of sodium cyanide in the carbon bodies causing disposal problems with the spent potlinings. The Environmental Protection Agency has listed spent potlinings as a hazardous material because they contain cyanides. Thus, it will be seen that there is a great need for a carbonaceous cathode that is wettable with molten aluminum and is resistant to formation of cyanide.
In U.S. Pat. No. 5,961,811, there is disclosed an improved carbonaceous material suitable for use as a cathode in an aluminum producing electrolytic cell, the cell using an electrolyte comprised of sodium containing compounds. The carbonaceous material is comprised of carbon and a reactive compound capable of suppressing the formation or accumulation of sodium cyanide during operation of the cell, and of reacting with one of titanium or zirconium to form titanium or zirconium diboride during operation of the cell to produce aluminum.
In attempts to provide aluminum wettable surfaces on carbon cathodes, application of titanium boride or zirconium boride has been suggested. These materials have been used as tiles to cover the cathode surface and are described in U.S. Pat. Nos. 3,400,061; 4,093,524; 4,333,813; and 4,341,611. However, these approaches have not been without problem. That is, the tiles and coatings tend to fall off after a short period of use, and this interferes with continued use of the cell. Also, coatings of titanium diboride have been applied in cement to the carbonaceous surface in U.S. Pat. Nos. 4,544,469; 4,466,692; 4,466,995; 4,466,996; 4,526,911; 4,544,469 and 4,624,766. EPO 0 021 850 suggests electroplating titanium diboride onto the carbon surface. U.S. Pat. No. 5,028,301 suggests deposition of a coating composed of titanium diboride and titanium carbide on cathode parts from supersaturated dissolved elements in electrowon aluminum. In a book entitled xe2x80x9cCathodes in Aluminum Electrolysisxe2x80x9d, 2nd edition, published 1994 by Aluminium-Verlag and authored by M. Sxc3x8rlie and H. A. Øye limited durability and cost of the material are cited as obstacles to effective industrial use.
Patent application (PCT) WO 00/29644 discloses wettable and erosion/oxidation resistant carbon composite materials. The materials are formed by mixing together finely divided quantities of TiO2 and B2O3 (or other metal boride precursors) to produce a precursor or mixture which is then mixed with at least one carbon-containing component to produce a carbon composite material that forms TiB2 (or other metal boride) in-situ when exposed to molten aluminum or subjected to heat-up of the cell. The invention also relates to carbon composite materials thus produced that may be used to form blocks (including sidewall blocks) for the construction of cathode structures (or coatings for such blocks) or may be used to prepare joint-filling and coating compositions for use in aluminum reduction cells, or protective coatings for instruments used with molten metals. However, when reactive materials are incorporated and mixed with the carbonaceous material, the resulting cathode block has compromised properties. For example, electrical conductivity is reduced or the block exhibits a greater electrical resistance detrimentally affecting the efficiency of the cell.
It will be seen that there is a great need for a method that permits the use of carbonaceous materials such as carbonaceous cathodes and blocks which is effective in promoting wetting with molten aluminum and is effective in preventing formation of undesirable compounds such as cyanide compounds during use of the cell to produce aluminum. Promoting wetting of the cathode greatly increases the efficiency of the cell. Preventing or reducing formation of compounds such as cyanide compounds minimizes post-treatment for the spent carbonaceous materials.
It is an object of the invention to provide an improved carbonaceous cathode for use in an aluminum producing electrolytic cell.
It is another object of the invention to provide a treatment for a carbonaceous cathode for an aluminum producing electrolytic cell, the treatment resulting in the cathode having improved molten aluminum wetting characteristics, leading to lower cell resistance, lower tendency to accumulation of sludge and decreased wear of the cathode surface.
Still, it is another object of the invention to provide a process for treating a carbonaceous cathode for use in an aluminum producing electrolytic cell, the treated cathode capable of reacting with a source of titanium or zirconium or like metal to form a metal boride on the cathode surface to promote improved wetting of the cathode with molten aluminum.
Further, it is another object of the invention to suppress or minimize air oxidation or air burning of cathode blocks during cell start-up and of exposed carbonaceous sidewalls in an electrolytic cell for producing aluminum.
These and other objects will become apparent from reading the specification and claims appended hereto.
In accordance with these objects there is provided a method of preparing carbonaceous blocks or bodies for use as a cathode in an electrolytic cell for producing aluminum wherein the cell contains an electrolyte and has molten aluminum contacting the cathode, the cathode having improved wettability with molten aluminum. The method comprises the steps of providing a carbonaceous block and a boron oxide containing melt. The carbonaceous block is immersed in the melt and pressure is applied to the melt to impregnate the melt into pores of the carbonaceous block. Thereafter, the carbonaceous block having boron oxide containing melt intruded into the pores is withdrawn from the melt, the boron oxide capable of reacting with a source of titanium or zirconium or like metal to form titanium or zirconium diboride during heatup or operation of said cell.